1. Field of the Invention
The present invention generally relates to spectrometry and, more particularly, to a linkage design used in a double crystal analyzer system.
2. Background Description
An “analyzer” is a spectrometer capable of measuring a single energy which can be scanned through a wide energy range. As used herein, “analyzers” go beyond simple frequency bandpass selection to include conditioning for topography, x-ray or neutron interferometry, x-ray or neutron standing waves, and phase selection. Analyzers covering the energy range from a few kilovolts and higher generally use single crystals of silicon or germanium as energy dispersive elements. Previously, Golovchenko et al. described an X-ray monochomator system for use with synchotron radiation sources (J. A. Golovchenko et al., Rev. Sci. Instrum. 52(4) April 1981). The Golovchenko system includes two separate Bragg reflecting crystals constrained by a mechanical linkage system. The design enables it to tune continuously in x-ray energies, while maintaining an exit beam of constant offset and direction relative to the incident beam.
FIG. 1 illustrates the “rigid right angle” concept of Golovchenko. Crystal positions are at a and b or a and b′. The concept of Golovchenko requires that the apex of the right angle Y must always have a vertical height exactly halfway between the height of the beam incident on the analyzer and the desired output beam height, and that the bar YX must always pass through some fixed position a at the incident beam height. The apex Y and second crystal at b are constrained to translate along parallel lines. For any given position of the apex Y along the midline MN, the rigid right angle must rotate about its apex Y in order that the length of YX intercept the fixed point a at the beam height.
Golovchenko indicated that the analyzer design acted as mechanical analogue computer generating proper crystal angles and displacements for the double crystal analyzer. With reference to FIG. 2, it can be seen that, depending on the planar spacing and angle of incidence θ, an energy will be selected by Braggs Law and the beam will be deflected by 2θ at a. Different positions of the apex Y along MN will yield different values of the angle of incidence θ, thereby permitting the system to have a mechanism to scanning through different selected energies. Thus, regardless of the energy chosen (as dictated by the horizontal position Y), the beam reflected from a will always intercept the bar YZ at the desired beam height. That is, the reflected beam, the bar YZ and the line PQ will always intercept each other at one point. This is proven by extending the line ZY to intercept the incident beam height at point c. The equality of incidence and reflection angles from the first crystal coupled with the congruence of triangles Yax and Yab yields the desired result.
FIG. 3 provides a summary of motions required for operation of the Golovchenko linkage. In the x-ray analyzer system, bearing positions of low energy setting are drawn in solid (XYZ angle) and those for a higher energy setting are dashed (X′Y′Z′ angle). To change energy, the rigid bar YX must slide through and rotate about point a. The linear bearing through which YX slides must therefore be mounted on a rotary bearing whose axis is at a perpendicular to the plane of FIG. 3. The first crystal is mounted directly over the linear bearing at a and perpendicular to YX. Another compound linear slide and rotary motion allows the apex Y to translate along rigid rod MN which is halfway between the incident beam height and exit beam height. Another bearing package slides on PQ. The combination contains two linear slide bearings joined by a rotary bearing. The second crystal mounts above the slide through which YZ passes and is oriented parallel to YZ.
P. L. Cowan et al., Nuc. Inst. Meth. 208:349–353 (1983), describes application of the Golovchenko et al. system for use in an ultra high vacuum (UHV) environment. The assembly is referred to as a “boomerang”, and similar to the Golvchenko et al. design, the cross roller slides forbid any relative rotation of the two crystals, and the Bragg angle of both crystals is changed by rotating the “boomerang”. Further, the linkage is completed by fixing one crystal axis and constraining the apex pivot and the second crystal axis to move along two equally spaced lined parallel to the incident radiation.
G. Jones et al., Rev. Sci. Instrum. 66(2) February 1995, describes a “Cowan type” double crystal analyzer based on the boomerang design which is used with advanced light source (ALS) that has a beam line covering the 1–6 keV photon-energy range. The design eliminates all of the bearings, and movement is accomplished using dovetail slides made of aluminum/bronze that slide on stainless steel bases.